The findings "suggest that transplanted dopamine neurons can remain healthy and functional for decades," study co-author Ole Isacson, of Harvard University and McLean Hospital in Boston, said in a news release from the journal Cell Reports. The study was published in the journal on June 5.

As explained by the study authors, the tremors and other symptoms experienced by Parkinson's disease patients are caused by the loss of dopamine-producing neurons (brain cells). Healthy dopamine-producing neurons can be transplanted into patients' brains, but there were questions about how long the transplanted cells would remain healthy.

This study included five Parkinson's patients who had received transplants of fetal tissue-derived dopamine-producing neurons four to 14 years ago. Their transplanted dopamine neurons appeared healthy and showed no signs of Parkinson's disease-associated deterioration.

The findings provide further support for stem cells as a source for transplant-ready dopamine neurons, the study authors said. Isacson noted that neuronal transplant has proven to be a durable treatment for many Parkinson's patients, with some improving for years without any need for standard medications.

However, he added that the therapy is still not common, with access so far limited to patients participating in certain clinical trials.

Isacson called the new finding "extremely encouraging," adding that the long life of the transplanted neurons bodes well "for advancing [the technique] as a restoration therapy for Parkinson's disease."